The invention relates to a camera tube having an electron source and a target. The target is, in operation, scanned on one side by an electron beam emanating from the source. The target comprises a selenium-containing vitreous layer on the side scanned by the electron beam and a signal electrode; on another, radiation-receiving side. The vitreous layer also contains tellurium, in a concentration which varies in the direction of the thickness of the selenium-containing layer, and arsenic.
A camera tube of the above-described kind is disclosed in British Patent Specification No. 1,135,460.
A problem with vitreous selenium layers is that they are comparatively insensitive to long wave length radiation. Therefore, additions such as tellurium are often used which improve sensitivity.
In addition it is important inter alia in order to achieve good operation of the camera tube, that there is good blockage against the injection of holes from the signal electrode into the selenium-containing layer so as to minimize the dark current and the lag. The dark current and the lag, however, may be considerable if sensitivity-improving substances are added in high concentrations. On the other hand, if sensitivity-improving substances are added in low concentrations, an annoyingly high operating voltage may be necessary and only a moderate sensitivity to long wave length radiation may result.
The selenium-containing layer with additions may be separated from the signal electrode by a vitreous layer of pure selenium so that there is good blockage between the signal electrode and the selenium-containing layer with additions. The disadvantage of this solution, however, is that by the use of a layer of pure selenium the glass stability is reduced (i.e. the layer has an increased tendency to crystallize) and the lag increases as compared to the above-mentioned layers having a low concentration of sensitivity-improving additions.
The above-described detrimental effect of high concentrations of sensitivity-improving additions also occurs if the additions are present in a concentration which decreases continuously from the radiation-receiving side to the side to be scanned.
In addition the glass stability of the camera tubes described in the British Patent Specification is low as a result of the low concentration of glass-stabilizing additions, for example arsenic.